Servo system with saturable core component



y 1951 J. P. PALMER SERVOSYSTEM WITH SATURABLE CORE COMPONENT 2 Sheets-Sheet 1 Filed Dec. 28, 1948 Tw T mm 3nvemor JAMES P. PAL Mi? (Ittozgneg July 3, 1951 J. P. PALMER SERVO SYSTEM WITH SATURABLE CORE COMPONENT 2 Sheets-Sheet 2 Filed Dec. 28, 1948 won Ihwemor m M a P P m m J 5 e n r 1 Patented July 3, 1951 saavo SYSTEM wrrn SATURABLE cons COMPONENT James P. Palmer, Ames, Iowa, assignor to General Precision Laboratory Incorporated, a corporation of New York Application December 28,1948, Serial No. 67,743

This invention pertains to servo systems containing saturable reactors or saturable transformers.

By saturable reactor is meant an electromagnetic reactor containing in addition to its'alternating current winding a direct current winding capable of producing enough ampere turns of magnetizing force to magnetize the core to saturation. Addition of a secondary alternating current winding to such a reactor converts it into a 'saturable transformer. It is preferable to arrange the windings of both devices so that no in-. duction existsbetween any direct current winding and any alternating current windingas'is well understood in the art.

In this disclosure saturablereactors and sat urable transformers, grouped together, will be termed saturable electromagnetic; devices."

It is possible to rectifya portion of the current flowing in the output alternating winding of. any saturable electromagnetic device by meansjof a rectifier connected in series with the winding,

Claims. (Cl. 313-28) lar system the introduction of viscous friction or its equivalent can be made to damp out oscillation. Therefore, by increasing the frequency. by employing friction-equivalent elements, bythe judicious use of positive feedback and by combination of the saturable electromagnetic devices with amplifiers of other types, servo systems containing saturable electromagnetic devices can be made to operate. In many applications the employment of saturable electromagnetic devices is to be preferred to the employment of equivalent electronic devices, particularly where large power is concerned in which-case the auxiliary equipment required when electronic tubes are used becomes cumbersome and expensive and requires use of high electrical potentials.

A purpose of the present invention therefore,

I is to provide a servo system utilizing saturable and to apply the rectified current to a.n a.d di

tional winding in a direction to aid the-magnetization produced by the direct current:controlfwinding. This action constitutes positive feedback, and thefeedback current, proportional in amount to the current in the primary alterna-t-f electromagnetic devices thus producing a simpler and more economical device than has heretofore been available.

Strict static proportionality between the output of a servo system and its input resulting in complete elimination of zero error can be attained even theoretically only when the servo amplifier has infinite gain, and in practice the employment of an amplifier having infinite gain a does eliminate all static follow-up error within ing winding, assumes part of the workof-mag netization which otherwise would be done-by the controlling directcurrent. In fact,the detakes over almost all of the work permitting very small direct currents to control a. relatively large alternating current. It can be shown that the limit to the maximum amount of stable feedback which can be used is reached when the number of ampere turns on the feedback winding equals that of the alternating current primary winding.

In many types of servo systems the time lags inherent in some of the components may produce oscillations in a servo loop. This possibility places strict requirements on servo design and dictates attention to the reduction of time constants. In the past, it has often been assumed that the large electrical time constants associated with iron-cored coils of many turns preclude the use of saturable electromagnetic devices in servo systems, but experimentsunderlying the instant invention have shown this assumption to be false. It has been found that the time lag of a saturable electromagnetic device is inversely proportional to the frequency of the alternating current employed and it is well known that for a given time lag in any particusign may be made such that the feedback current practical design limits.

Another purpose of the present invention 1 therefore, is to utilize an amplifier having infinite gain in combination with a saturable electro magnetic control device.

This invention will be more readily understood from the following detailed description considered together with the attached drawings in which:

Figure l is a diagrammatic illustration of one form of the invention utilizing a saturable transformer.

Figure 2 is a fragmentary diagram illustrating a modification of'the invention utilizing a saturable reactor.

Referring now to Fig. 1, two similar resistances II and I! having sliders l3 and M are connected in series to form a difierential potentiometer. The sliders l3 and I4 are connected together by two equal series resistors l6 and I! the midpoint l8 of which is connected to the control grid IQ of a triode 2i which together with the triode 22 comprises a direct, current differential amplifier stage. By proper selection of values for the circuit parameters of this stage and the particular circuit incorporated therein, this stage may be '.made to have infinite gain as will be explained more fully hereinafter.

The control grid 23 of the triode 22 derives its bias potential from a voltage divider circuit which consists of the resistor 29, the lower portion of the potentiometer-38 and the resistor 39, the series circuit formed by these elements being connected between a source of negative potential indicated diagrammatically by the terminal 31 and a positive potential point comprising the connection of the resistor 39 to the plate 4| of the tube 2 I.

The resistor 29 and potentiometer 38 together with resistor 23 are connected in shunt to the resistors H and I2 forming a bridge circuit therewith and this circuit is connected to ground through a resistor 21. A potential drop is therefore imposed across this bridge circuit and the slider 24 acts as a means for initially adjusting the difierential balance of this bridge circuit.

The slider I3 is operated by any desired input mechanism, the exact nature of which depends on the application to which the invention is put, through the medium of a shaft 32.

The infinite gain'amplifier composed of triodes 2| and 22 includes a common cathode resistor 36 connected to the negative potential source 31 and plate resistors 33 and 34 connected to a source of positive potential. The resistor 39 connected between the plate 4| of the tube 2| and the grid 23 of the tube-22, constituting a portion of the divider circuit for imposing the bias potential on the grid 23, coacts with the other circuit elements to produce infinite gain in the following manner:

Let it be assumed that the input displacement shaft 32 is pushed upward a finite amount, moving slider I3 upward. This change in the potentiometer adjustment makes conductor 3| more positive and also makes junction l8 more positive by a smaller amount. The grid l9 of tube 2| becomes more positive; more plate current flows and the potential drop in the cathode resistor 36 increases, raising the potential of the cathode 42 of the tube 22 and thus reducing its plate current flow. The increase of current flow in the plate resistor 33 reduces the potential of the plate 4| and of the grid 43 of tube 44 of the followin stage, while the decrease of current flow in the plate resistor 34 increases the potential of its plate 41 by an equal amount and likewise of the grid 48 of tube 46. As so far described, the potential of the grid 23 has not changed. However, actually, because of the drop in potential of the plate 4| communicated through resistor 39, the potential of the grid 23 does drop somewhat, still further reducing the current flow through the cathode resistor 36 due to tube 22 and thus making the cathode 49 of the tube 2| more negative. This is equivalent to making the grid l9 more positive relative to the cathode, and by properly proportioning the resistor 39 to the voltages and other resistances, the amountby which the grid I9 is made in effect more positive by making the cathode more negative can be made exactly equal to the small initial grid change assumed as the input signal. Therefore a very small, even infinitesimal change of potential at the grid l9 will produce a large stage output. When there is any input signal at all, even though it be so small as to be unmeasurable, there will be an output signal limited in size only by the circuit constants. In this practical sense then as well as in the theoretical sense, the amliflcation is infinite and such an amplifier is 4 T here called an infinite gain amplifier. As here used in a servomechanism this circuit is stable, because the negative feedback servo loop will serve to damp out any oscillations which would otherwise persist.

The plate 5| of tube 44 of a second electronic amplifier stage is connected to the control coil 52 of a saturable transformer 56, and the plate 62 of tube 46 is similarly connected to the control coil 51 of asecond saturabletransformer 6|. The other terminals of these control windings are connected to a terminal 63 representing a source of positive potentials Each saturable transformer has a primary winding, 53 and 53, and a secondary winding 54 and 59 in inductive relation to its primary in addition to the direct current control winding 52 and 51. The latter winding is so placed as to be capable of magnetizing the transformer core yet not to be inductively coupled to the other windings on the core as is well understood in the art. The primary windings 53 and 58 are connected in series across a source of alternating potential represented by terminals 64-64, which may for example, be at volts and 60 cycles. The secondary windings 54 and 59 are connected in series opposed to the two terminals of a coil 66 of a two-phase low-inertia motor 61. Being in opposed series, the instantaneous potentials of the secondary windings are opposed, which is equivalent to saying that they are out of phase with each other. The secondary windings are also shunted by a condenser 66, the purpose of which is to provide a path for harmonic potentials and to reduce the amount of such harmonics flowing in coil 66. The other winding 69 of the two-phase motor is connected in series with a 90-degree phase-shifting condenser 1| across the power terminals 54. The direct current control windings are each shunted by a protective resistor 12 and 13, to provide discharge paths for the stored energy in the control coils on any sudden reduction of current through them, thus preventing flashover in the tubes 44 and 46 and consequent injury to them and other components. The static current flow through the control coils 52 and 51 is that of the respective tube plate currents, and is adjusted'by slider 14 on potentiometer 16 controlling the negative bias on the grids 43 and 48.

The circuit being balanced when no signal is impressed thereon, equal plate currents flow in the control windings 52 and 51, partly and equally magnetizing their respective cores and somewhat reducing the magnetic permeability of each. Equal and moderate magnetizing currents will fiow in the primary windings 53 and 58, and the inductive reactance drops across them will be equal. Accordingly equal potentials will be induced in the secondary windings 54 and 59, and since they are connected in opposition, no potential results across the motor winding 66. However, when an input signal occurs more current flows in one control coil, for example coil 51, and less in the other coil 52 as before stated. This increases the magnetization and decreases the permeability and induction in the core of the transformer 6| and produces an opposite change of like amount in transformer 56. Consequently the potential across the primary winding 58 drops, while that across the primary winding 53 rises an equal amount, and the secondary potentials are changed in a similar manner, that of the secondary 54 becoming larger than that of secondary 59. Current then flows because a net potential is produced across the motor coil 66.

This potential will be approximately 180 displaced in phase from that across the primary winding 53 and at the terminals 64, therefore, the resulting current will lead that in the winding 88 by approximately 90 and the motor will rotate, let it be assumed, clockwise. Should the input signal be reversed, the current in the control coil will exceed that in the coil 52 and the motor will rotate in the counterclockwise direction for similar reasons.

The dashed line 11 represents a mechanical negative feedback connection from the shaft of motor 81 through reduction gearing I5 to the slider I4 of potentiometer I2, the connection being in such sense that the rotation of the motor tends to restore the potential of the grid I8 of tube 2I to that which it had before reception of the signal. That is, if the signal consisting of a motion of the input shaft 32 were upward in the drawing and caused an increase of potential at the grid I8, the feedback signal would push the slider I4 downward, again decreasing the potential of the grid I8. Since the gain of the first stage is infinite, the fed back motion will continue until the potential of the grid I8 attains its exact initial potential, that is, zero change in error signal at the grid I8 occurs.

The rectangle 83 represents a mechanical load of any type, such as a dial, a generator, or another servo, which is to be operated by the servomechanism.

In most servomechanisms time lag must be minimized to eliminate oscillation and hunting and the greater part of the lag is usually in the mechanical parts of the loop. The condensers I8 and I8 connected in parallel to the resistors 8I and 82 between the outputs of tubes 2I and 22 and the inputs of tubes 44 and 46 neutralize part of this lag, producing phase advance in the two sides of the difierential circuit under the conditions of changing or recurring signals.

The operation of the servomechanism as thus far described would be likely to produce overshooting or hunting of the feedback slider I 4. To prevent such improper operation a condenser 84 is connected in shunt to the resistor IT. A varying potential at slider I4 will pass directly through this condenser 84 to the grid I8, so that the resistor I1 is efiectively short-circuited. To

the grid I8 it will then appear that the feedback K slider has reached its final position when in fact it has traveled but half the necessary distance and, controlled by the time constant of the condenser 84 and. the resistor II, the grid will approach its ultimate potential at a continuously diminishing rate, so that overshooting or hunting will not occur.

Figure 2 illustrates the employment of saturable reactors in place of the saturable transformers in the servomechanism of Fig. 1. Either type of saturable device can be used in such a servomechanism, the transformer afi'ording more freedom in design, but the reactor is cheaper and utilizes a simpler circuit. Fig. 2 also illustrates the employment of a saturable electromagnetic device containing positive feedback.

In Fig. 2 only so much of the circuit as differs from that disclosed in Fig. 1 is illustrated, namely, that portion of the circuit connected to the outputs of tubes 44 and 46 which as in Fig 1 constitute the second stage of the amplifier circuit. The plate SI of tube 44 is connected through the conductor 81 to the coil 88 of a reactor" and the plate 62 of tube 46 is connected through the conductor 8| to the coil 82 of a. re-

actor 83. The other terminals of coils 88 and 82 are connected to a source of positive potential represented by terminal 84. Resistors 88 and 8! shunting coils 88 and 92 respectively, are pro- 'vided to protect the tubes 44 and 4.8 against inductive surges if the currents through the coils should be suddenly reduced. The reactor 88 is may be at 115 volts 60 cycles, the winding 88- being in series with motor field coil I01 and winding 88 being in series with motor field coil I04. One terminal of each field coil is connected to the power source terminal I03 while the other terminals -I I3 and H4 of the field coils are joined by a -degree phase-advancing condenser I08. The common terminal I88 of the reactor winding is connected to power terminal I02. A full wave rectifier I08 is connected in series between the reactor coil 88 and the motor field coil I01 and another similar rectifier I I6 is connected between the reactor coil 89 and the motor coil I04. The direct output current from each rectifier supplies a feedback winding, III and I I2, on its associated reactor core. These windings are arranged in noninductive relation with their respective alter- ;nating current windings, and are polarized to aid the magnetization effected by the direct current control windings 88 and 82. The number of turns on each feedback winding is not greater than the number of turns on its associated alternating current winding.

Under no-signal conditions as described in connection with Fig. 1, equal currents flow in the plate circuits of the discharge tubes 44 and 48 and in their control coils 88 and 82. These currents determine the permeabilities of their respective reactor cores and hence the amount of induction and back electromotive force efiective in the two alternating current coils 88 and 88, results in equal potential drops across them. The currents through them will, therefore, be equal as will the currents through the series motor field coils I04 and I01, resulting in zero net motor field flux. But when an input signal exists as described in connection with Fig. 1 the discharge tube plate currents become unbalanced, for instance, the current in conductor 81 may become greater than that in conductor 9|. The permeability of the core of reactor 88 will thereby be lowered, lowering the magnetic induction and the back electromotive force, resulting in a reduced potential drop across the coil 88 and a greater current through it. A corresponding reduction in the current through the coil 88 will simultaneously take place for similar reasons. The increased current through and potential across the motor field coil I01 compared with that of coil I04 will produce a net field flux, while the potential between the terminals H3 and II 4 will produce a current through the condenser I06, energizing the motor field winding I 04 phased 90 ahead 01' the energization of the field I 01 and the motor will rotate. A reversal of the input signal resulting in a decrease of the current in conductor 81 and increase in conductor 8I will result in an opposite motor rotation.

Meanwhile, when the current through the reactor coil 88 increases, the current through the rectifier I08 will be increased, increasing the amount of direct current flowing in the feedback coil 89. Since the effect of this coll aids that of the control coil '8, the control effect is increased. Similarly, reduction in the alternating current through the rectifier H6 will reduce the feedback current in coil H2, enhancing the reduction in the control current in the coil 92.

The motor I! is connected mechanically, as represented by dashed line I", to a load and to a negative feedback slider as described in connection with Fig. 1, constituting a servo feedback loop, and operating as before described to deliver an output displacement theoretically equal and in practice equal within limits of manufacturing error to the input displacement.

What is claimed is:

1. In a servo system, means for producing a first potential proportional to an input quantity, means for producing a second potential, a device for varying said second potential, means for opposing said potentials to produce an error signal proportional to the unbalance thereof, a direct current differential amplifier having said "error signal" impressed on its input and having a first output circuit the potential of which varies directly as a function of said error signal" and a second output circuit the potential of which varies inversely as a function of said "error signal a first saturable electromagnetic device having a control winding connected in said first output circuit, a second saturable electromagnetic device having a control winding connected in said second output circuit, output windings for each of said saturable electromagnetic devices connected in series opposed relation, a twophase motor, means including said series opposed output windings controlling the speed and direction of rotation of said motor and means for adjusting said first mentioned device in accordance with the rotation of said motor.

2. A servo system according to claim 1 in which said amplifier includes means for advancing the phase of the signal amplified thereby whereby time lag of response is greatly reduced.

3. A servo system according to claim 2 in which said amplifier is provided with means to produce infinite gain thereby over its operating range.

4. In a servo system means for producing a first potential proportional to an input quantity, means for producing a second potential, a device for varying said second potential, means for opposing said potentials to produce an error signal" proportional to the unbalance thereof, a differential amplifier having said error signal" impressed on its input and having a first output circuit the potential of wh ch varies directly as a function of said error signal and a second output circuit the potential of which varies inversely as a function of said error signal," a first saturable transformer having a control winding connected in said first output circuit, a second saturable transformer having a control winding connected in said second output circuit, primary windings for each of said saturable transformers energized by an alternating current, secondary windings for each of said saturable transformers connected in series opposed relation, a two-phase motor having a first field coil energized by a source of phase-shifted alternating current and a second field coil connected in circuit with said secondary windings and means for adjusting said first mentioned device in accordance with the operation of said motor.

5. A servo system according to claim 4 in which said amplifier includes means for advancing the phase of signal amplified thereby, whereby time lag of response is greatly reduced.

6. A servo system according to claim 5 in which said amplifier is provided with means to produced infinite again thereby over its operating range.

'7. In a servo system means for producing a first potential proportional to an input quantity, means for producing a second potential, a device for varying said second potential, means for opposing said potentials to produce an error signal" proportional to the unbalance thereof, a diiferential amplifier having said error signal impressed on its input and having a first output circuit the potential of which varies directly as a function of said error signal" and a second output circuit the potential of which varies inversely as a function of said error signal" a first saturable reactor having a control winding connected in said first output circuit, a second saturable reactor having a control winding connected in said second output circuit, alternating current windings for each of said saturable reactors connected in series opposed relation, a motor having a pair of field coils displaced electrical degrees with respect to each other with their adjacent ends connected together and to one terminal of an alternating current supply source and their remote ends connected to the remote ends of the saturable reactor alternating current windings, a circuit connecting the adjacent ends of said alternating current windings and the remaining terminal of said alternating current supply source, a condenser connected in parallel to said motor field coils and means for adjusting said first mentioned device in accordance with the operation of said motor.

8. A servo system according to claim 7 in which said amplifier includes means'for advancing the phase of the signal amplified thereby, whereby time lag of response is greatly reduced.

9. A servo system according to claim 8, in which said amplifier is provided with means to produce infinite gain thereby over its operating range.

10. A servo system according to claim 9 in which each of said saturable reactors is provided with a third winding each of which is energized by a direct current proportional to the alternating current flowing through its associated alternating current winding.

11. In a servo system, means for producing a first potential proportional to an input quantity, means for producing a second potential, a device for varying said second potential, means for opposing said potentials to produce an error signal proportional to the unbalance thereo, a differential amplifier comprising first and second tubes having their cathodes connected together and to a source of potential through a common cathode resistor, means for impressing said error signal on the control electrode of said first tube, a voltage divider network connected between the plate of said first tube and a terminal of lower potential, a connection between the control electrode of said second tube and an intermediate point on said voltage divider network, said common cathode resistor and said voltage divider network being so proportioned that a variation in error signal results in an equal and opposite change in the potential of the oathode of said first tube, a saturable electromagnetic device having a control winding actuated by said amplifier output and motor means controlled by said saturable electromagnetic device for adjusting said first mentioned device to reduce said error signal to zero.

12. In a servo system, means for producing a first potential proportional to an input quantity, means for producing a second potential, a device for varying said second potential, means for opposing said potentials to produce an error signal proportional to the unbalance thereof, a differential amplifier comprising first and second tubeshaving their cathodes connected together and to a source of potential through a common cathode resistor, means for impressing said error signal" on the control electrode of said first tube, a voltage divider network connected between the plate of said first tube and a terminal of lower potential, 9. connection between the control electrode of said second tube and an intermediate point on said voltage divider network, said common cathode resistor and said voltage divider network being so proportioned that a variation in error signal results in an equal and opposite change in potential of the cathode of said first tube, a first output circuit connected to said differential amplifier whose potential varies directly as a function of said "error signal" a second output circuit connected to said differential amplifier whose potential varies inversely as a function of said error signal, a first saturable transformer having a control winding connected in said first output circuit, a second saturable transformer having a control winding connected in said second output circuit, primary windings for each of said saturable transformers energized by an alternating current, secondary windings for each of said saturable transformers connected in series opposed relation, a two-phase motor having a first field coil energized by a source of phase-shifted alternating current and ror signa on the control electrode-of said first tube, a voltage divider network connected between the plate of said first tube and a terminal of lower potential, a connection between the control electrode of said second tube and an intermediate point on said voltage divider network, said common cathode resistor and said voltage divider network being so proportioned that a variation in error signal results in an equal and opposite change in potential of the cathode of said first tube, a first output circuit connected to said differential amplifier whose potential varies directly as a function of said error signal," a second output circuit connected to said differential amplifier whose potential varies inversely as a function of said error signal, a first saturable reactor having a control winding energized by said output circuit, a second saturable reactor having a control winding energized by said second output circuit, alternating current windings for each of said saturable reactors connected in series opposed relation, a motor having a pair of field coils displaced 90 electrical degrees with respect to each other with their adjacent ends connected together and to one terminal of an alternating current supply source and their remote ends connected to the remote ends of the saturable reactor alternating current windings, a connection between the adjacent ends of said alternating current windings and the remaining terminal of said alternating current supply source. a condenser connected in parallel to said motor field coils and means for adjusting said first mentioned device in accordance with the opera tion of said motor to restore the balance between said first and second potentials and to reduce said error signal to zero.

15. A servo system according to) claim .14 in which said differential amplifier includes a phase advancing circuit whereby changes in error siga second field coil connected in circuit with said secondary windings and means for adjusting said first mentioned device in accordance with the operation of said motor to restore the balance between said first and second potentials and to reduce said "error signal to zero.

13. A servo system according to claim 12 in which said differential amplifier includes a phase advancing circuit whereby changes in error sig- Number nal" are anticipated and the time lag of response greatly reduced. I

JAMES P. PALMER.

REFERENCES CITED The following references are ofrecord the sures PATENTS Name Date 2,208,623 Bond July'23, 1940 2,333,393 Ryder Nov. 2, 1943 2,338,423 Geyger Jan. 4, 1944 2,355,567 Sparrow Aug. 8, 1944 2,414,936 Edwards et a1 Jan. 28, 1947 2,435,926 Krupick Feb. 10, 1948 2,449,859 Penfold et a1. Sept. 21, 1948 2,450,084 Emerson Sept. 28, 1948 OTHER REFERENCES D. C. Operated Servo Amplifier, by Sidney Wald, August 1848, Radio News, pp. 14, 15 and 28. 

